1. Field of the Invention
The present invention relates to injection molding machines, and, more particularly, to a mechanical interlock mechanism for the safety cover of the die closing unit of an injection molding machine, intended to prevent access to the injection molding die, when the die closing unit moves from the open position to a closed position.
2. Description of the Prior Art
It is a general safety requirement for injection molding machines to include a safety gate or safety cover which encloses the area of its injection molding die and which, in order to prevent injury to a machine operator, includes an interlock device which blocks the operation of the machine, when the safety cover is not in its closed position.
From the prior art in this field are known three different types of safety cover interlock devices: The interlock device may be of the electrical type, using one or several limit switches which, when the safety cover is moved from its closed position, interrupt the drive motor circuit and/or block the operation of other electrical components of the machine; it may be of the hydraulic type, using one or more hydraulic shutoff valves which, when actuated by cam surfaces which move with the safety cover, block the operation of the hydraulic drive circuit of the die closing unit; or it may be of the mechanical type, using a motion arresting mechanism which mechanically blocks the die closing movement by interposing an arresting member, when the safety cover is in its closed position.
Experience has shown that, in spite of the posting of safety rules and warnings, and, in spite of the known risk of potentially great personal injury, some operators and mechanics will attempt to inactivate and circumvent any safety interlock device, if the possibility to do so exists. It has therefore already been suggested to utilize multiple safety interlocks--preferably of all three above-mentioned types--and to arrange these interlock devices in a tamper-proof manner, so that even an experienced mechanic will not be able to defeat their purpose.
In U.S. Pat. No. 4,113,414 is disclosed such a tamper-proof safety cover for the die closing unit of an injection molding machine with multiple tamper-proof safety interlock devices. The displacement of the safety cover away from its closed position actuates electrical limit switches as well as a hydraulic shutoff valve, all of them being inaccessibly enclosed within a sealed housing. A separate mechanical motion arresting device blocks the advance of the movable die carrier towards its closed position, when the safety cover is not in the closed position. This mechanical arresting device consists of a push rod which extends axially forwardly from the movable die carrier plate, laterally outside of the injection molding die, and through a bore of the stationary die carrier plate. That bore is interrupted by a larger cross bore which accommodates a spring-loaded tumbler with a bore portion which is aligned with the die carrier bore, only when it is moved into that position by a control rod which moves with the safety cover. This control rod is retracted, when the safety cover is displaced from its closed position, thus allowing the spring-loaded tumbler to move into a position in which it blocks the advance of the push rod, thereby arresting the closing movement of the movable die carrier plate. A similar tumbler-type mechanical interlock mechanism is disclosed in German Pat. No. 15 29 779. These devices have the disadvantage that they create canting forces in the die closing unit and, if duplicated to be arranged on opposite sides of the injection molding die, present a restriction to the access to the latter.
In U.S. Pat. No. 3,386,133 is suggested another mechanical safety cover interlock mechanism, which is arranged in a knee-lever-type die closing unit. This mechanism includes two transversely spaced guide rods which extend axially rearwardly from the movable die carrier plate and reach through and beyond two bores of a stationary thrust plate which serves as a support for the knee lever mechanism. One of the protruding guide rod portions is provided with sawtooth-like notches which are arranged to cooperate with an arresting pawl. The latter executes a pivoting movement in response to a cover-connected cam and linkage mechanism, in such a way that the pawl engages a guide rod notch, when the safety cover is moved from its closed position. The engaged arresting pawl blocks the advance of the guide rod and of the movable die carrier plate.
A very similar interlock device is suggested in U.S. Pat. No. 3,728,057. It differs from the mechanism just described primarily in the manner in which longitudinal position adjustments of the stationary thrust plate are automatically compensated for with respect to the cover-actuated pawl control mechanism. However, neither of these two devices is tamper-proof, the rod-blocking action of the arresting pawl being easily defeated by simply holding the pawl in a raised position, out of engagement with the guide rod. An additional disadvantage of these devices is that they utilize only one of the two guide rods for cooperation with the arresting mechanism and that the latter is subject to elevated localized pressures which occur under off-center abutment conditions.